Gasification of carbonaceous material

ABSTRACT

A grate assembly ( 10 ) for a gasifier for gasifying carbonaceous material producing ash includes an upper rotatable grate component ( 11 ) and a lower rotatable support structure ( 12 ) fastened to the upper rotatable grate component ( 11 ) by a plurality of removable fasteners ( 15 ) acting to prevent vertical displacement of the upper rotatable grate component ( 11 ) and the lower rotatable support structure ( 12 ). The lower rotatable support structure ( 12 ) is configured to be drivingly rotated about a common vertical axis of rotation ( 21 ) shared with the upper rotatable grate component ( 11 ). The assembly ( 1 ) further includes at least two key torque transmitters ( 16 ) spaced angularly relative to the common axis of rotation ( 21 ) and engaging the upper rotatable grate component ( 11 ) and the lower rotatable support structure ( 12 ) to transfer torque from the lower rotatable support structure ( 12 ) to the upper rotatable grate component ( 11 ) when the lower rotatable support structure ( 12 ) is driven.

THIS INVENTION relates to gasification of carbonaceous material. In particular, the invention relates to a grate assembly for a gasifier for gasifying carbonaceous material and to a gasifier for gasifying carbonaceous material.

For some gasifiers, such as the Sasol® FBDB™ gasifier, it is necessary intermittently or continuously to load and unload particulate material, such as particulate carbonaceous material (e.g. coal) and ash. Thus, for example, in the case of a pressurized gasifier (e.g. a fixed bed dry bottom gasifier), particulate carbonaceous feedstock, e.g. coal, is loaded into a gasification chamber of the gasifier to form a bed (in the case of a fixed bed gasifier), and gasified at elevated temperatures and pressures, and after gasification any remaining ungasified material is removed from the gasifier as ash via a rotatable grate assembly and an ash lock.

The grate assembly has two main mechanical functions. Primarily it is used to extract ash from the bottom of the gasification chamber, but it is also used to crush and remove any ash agglomerates (clinkers) that may be formed in the gasification process. The grate assembly is also used to distribute gasification agent (typically a mixture of steam and oxygen) into the bed of carbonaceous material.

The grate assembly comprises of a number of components, principally an upper roughly conical-shaped rotatable grate component, a lower rotatable support structure fixedly connected to the upper rotatable grate component at an upper periphery of the rotatable support structure and a ring gear connected at a lower periphery of the rotatable support structure. A lowermost stationary support structure is provided on which the rotatable components are supported. In use, the grate assembly is rotated when the ring gear which is drivingly connected to the lower rotatable support structure is driven via a motor and gearbox assembly which turns the rotatable components of the grate assembly.

In the grate assembly of the prior art, the rotatable grate component is fixedly connected to the rotatable support structure with a plurality of bolts located in corresponding bolt holes and a single key torque transmitter is located in a corresponding keyway. The connecting bolts are intended to prevent vertical separation of the upper rotatable grate component from the lower rotatable support structure. In use, the key functions as a torque transmitter between the adjacent components in order to transmit the drive force from the ring gear, through the lower rotatable support structure to the upper rotatable grate component. In the grate assembly of the prior art, a single key is used as a torque transmitter. The key is designed to withstand the entire radial load or shear force experienced by the grate assembly in an upset condition. It is commonly believed in the prior art that, in use, a single key will always seat before other keys and will carry the entire radial load or shear force and therefore the use of multiple keys is regarded as superfluous.

It has been found that, in use, the connecting bolts and, on occasion, the key itself fail in upset conditions where the free rotation of the grate assembly is obstructed. Such an upset condition may occur where hard ash agglomerates (clinkers) are present and become lodged in an ash discharge annulus defined between the rotatable grate component and a wall of the gasification chamber. In such an instance, the free rotation of the upper rotatable grate component is restricted, whilst the motor and ring gear continue to rotate and act on the lower rotatable support structure. It will readily be appreciated that in such an event the torque on the rotatable grate component increases and that the load transferred from the gearbox, though the ring gear to the lower rotatable support structure and to the upper rotatable grate component is carried by the key torque transmitter. Failure of the connecting bolts is thus an unexpected failure since the key is designed to resist the radial forces associated with an upset condition wherein the free rotation of the upper rotatable grate component is hindered. In use, the connecting bolts are in effect protected by the key and are accordingly not designed to carry any of the tangential shear loads associated with upset conditions.

When the connecting bolts shear, the upper rotatable grate component and the lower rotatable support structure separate and the grate assembly can no longer remove ash from the gasifier. This results in the shutdown of the gasifier which is clearly undesirable as it may severely impact on production levels and income generated from the gasifier. The prior art design thus results in inefficient operation and reduction of this inefficiency would be beneficial.

In order to mitigate functional failure arising from failure of the connecting bolts, attempts were made to use larger and/or stronger connecting bolts. Attempts were also made to weld the key to the two interfacing components (i.e. the rotatable grate component and the rotatable support structure) as it was firmly believed that the displacement of the key out of position was the main cause leading to failure of the connecting bolts. Both approaches have, however, proven in practice to be unsuccessful in mitigating functional failure of the grate assembly as described hereinbefore.

Investigations have revealed that the failure of the grate assembly in upset conditions is partly due to a design flaw in the manner in which the connecting bolts and key torque transmitter connect the rotatable grate component and the rotatable support structure. Surprisingly, it has been found that, taking into account the dynamic mechanical loads in the grate assembly during an upset condition, a relative movement between the rotatable grate component and the rotatable support structure is introduced, i.e. one component (typically the rotatable support structure) continues to rotate while the other component in the assembly (typically the rotatable grate component) remains stationary, its rotation being hindered by the ash agglomerates trapped in the ash discharge annulus. In this event, a relative rotation around the key torque transmitter occurs, i.e. rotation of the rotatable grate component is hindered by the ash agglomerate(s) in the ash discharge annulus and it consequently pivots around the key torque transmitter.

In operation it is crucial that the axis of rotation of the two connected rotatable components coincides. Relative movement of the adjacent components around the key leads to a displacement of the connecting bolts relative to the corresponding bolt holes and of the key relative to the keyway. This leads to a displacement of the axis of rotation of one of the rotating components. Displacement of the axis of rotation leads to a progressive increase of the shear stresses on the bolts, and ultimately failure of the bolts. With the bolts not able to prevent the relative movement of the two adjacent components, the rotatable components of the grate assembly separate leading to a functional failure of the assembly.

In the course of the investigations, it was discovered surprisingly that the connecting bolt failure was not due to insufficient strength of the connecting bolts in the vertical plane (i.e. substantially parallel to the axis of rotation of the grate assembly), but due to misalignment of the rotatable grate component and rotatable support structure in the horizontal plane (i.e. in a plane substantially perpendicular to the axis of rotation) due to the relative rotation of the components around the key torque transmitter.

WO 2006/061738 attempts to overcome the problem of segregation of particulate coal feedstock fed into a fixed bed coal gasifier which results in gasifier operational instability and “hot spots” as a result of preferential gas flow through a non-homogenous coal bed so produced. This document discloses a fixed bed gasifier and a method of operating a fixed bed gasifier to overcome the aforesaid problems. The gasifier has a static coal distribution device inside a gasification chamber of the gasifier and a rotatable grate having at least one upwardly projecting finger above an ash outlet of the gasification chamber. The combination of the static coal distribution device and the rotatable grate component with projecting finger are claimed to improve the radial and axial distribution of coal and ash particles in the fixed bed of coal, thereby to homogenise the bed and improve gasifier operation and stability. WO 2006/061738 describes two alternative grate assemblies which are suitable for accomplishing its stated objective. Nowhere in WO 2006/061738 is there any mention of the problem of the misalignment between upper and lower grate components, relative rotation of upper and lower grate component around a key torque transmitter, or the functional failure of a grate assembly due to the shearing of the connecting bolts between upper and lower grate components. WO 2006/061738 is in fact directed at an entirely different problem than the problem the present invention is directed at.

CN 201762288 is a Chinese utility model and relates to a co-current, air-blown, fixed bed biomass gasifier. A prior art biomass gasifier described by CN 201762288 has a furnace, an ash chamber, a grate, supports and an ash discharge mechanism. The grate is a flat structure which is installed horizontally below the furnace. The grate has a plurality of holes extending through it to allow combustible gas and ash-slag to pass therethrough. CN 201762288 identifies that the problems associated with grates of the prior art include inter alia (i) that ash accumulates on the grate thus resulting in low gasifier efficiency, (ii) that ash blocks the holes in the grate thereby preventing combustible gas from exiting the furnace through said holes and inhibiting gas production capacity, (iii) that the holes in the grate lead to material leakage, and (iv) that existing grates are poorly suited to the gasification of a variety of types of biomass as the holes in the grate do not lend themselves to be adapted to the gasification of different types of biomass. Alternative gasifiers are thus proposed and illustrated in FIGS. 2-13 of CN 201762288. Although a component (13) is present in the gasifiers proposed in CN 201762288 and is termed a grate, it is not a rotatable grate in the conventional sense, but is in fact a stationary device positioned below the furnace. The grate (13) is designed to support a bed of biomass-derived ash and to allow combustible gas and ash-slag produced in the furnace to pass through holes in the grate (13), although apparently the majority of ash-slag is discharged through an ash-discharge annulus between the grate (13) and an inner wall of the furnace. An ash-agitating mechanism is provided to agitate ash collecting on top of the grate (13) and to push the ash into the ash-discharge annulus. The ash agitating mechanism is thus the only fully rotatable component in any of the biomass gasifiers described in CN 201762288. CN 201762288 further describes that it would be advantageous to be able to adjust the size of the holes in the grate (13) thereby to render the biomass gasifier adaptable to the gasification of various types of biomass. CN 201762288 discloses that one method of adjusting the size of the holes in the grate (13) is to provide an adjustment plate (19) having a substantially similar construction to the grate (13), i.e. being disc-like and having a plurality of holes disposed therethrough, installed coaxially with and underneath the grate (13) in the furnace. The adjustment plate (19) is rotatable to a limited extent so that, in a first position, the adjustment plate (19) is positioned relative to the grate (13) such that holes (13a) and holes (19a) overlap thus defining a plurality of channels extending through both components (13) and (19), or in a second or subsequent position, the adjustment plate (19) can be rotated slightly relative to the grate (13) such that the holes (19a) and the holes (13a) do not overlap (i.e. are offset), thus effectively reducing the size of the holes or channels extending through the grate (13) and adjustment plate (19). In this manner, the effective size of the holes (13a) in the grate (13) of the biomass gasifiers of CN 201762288 are adjustable such that various types of biomass can be gasified in the gasifier simply by rotating the adjustment plate (19) slightly relative to the grate (13). In order to limit the relative movement of the adjustment plate (19) relative to the grate (13), CN 201762288 describes a plurality of arcuate formations (29) projecting from the bottom surface of the grate (13) which slot into complementary arcuate depressions (28) in the adjustment plate (19) located beneath the grate (13). By fitting the projections (29) on the underside of the grate (13) into the complementary depressions (28) provided on an upper face of the adjustment plate (19) limited rotational movement of the adjustment plate (19) relative to the stationary grate (13) is allowed. The four arcuate projections (29) thus do not function as key torque transmitting members. The projections (29) and depressions (28) are merely included to guide, and to limit the range of, relative rotation between the stationary grate (13) and the rotatable adjustment plate (19). No torque whatsoever is transferred by the projections (29).

According to one aspect of the invention, there is provided a grate assembly for a gasifier for gasifying carbonaceous material producing ash, the grate assembly including:

an upper rotatable grate component; and

a lower rotatable support structure fastened to the upper rotatable grate component by a plurality of removable fasteners acting to prevent vertical displacement of the upper rotatable grate component and the lower rotatable support structure, the lower rotatable support structure being configured to be drivingly rotated about a common vertical axis of rotation shared with the upper rotatable grate component,

the assembly further including at least two key torque transmitters spaced angularly relative to said common axis of rotation and engaging the upper rotatable grate component and the lower rotatable support structure to transfer torque from the lower rotatable support structure to the upper rotatable grate component when the lower rotatable support structure is driven.

In this specification, the term “component” is intended to include an assembled component including more than one part, such as a rotatable grate component comprising a number of parts assembled to form the grate component.

It was surprisingly discovered that the introduction of a plurality of keys mitigates the functional failure experienced with the grate assembly of the prior art. The inclusion of a plurality of keys in accordance with the invention is thus not intended simply to strengthen the grate assembly as may be understood to be the case, but rather to maintain the geometrical integrity of the grate assembly.

The keys may be equiangularly spaced. Thus, for example, when only two keys are present, they may be about 180° apart, when three keys are present, they may be about 120° apart, and when four keys are present, they may be about 90° apart. It may however be that the arrangement of the keys is not symmetrical due to construction constraints (other components in the way, etc.) and the arrangement may thus be slightly offset, e.g. 170° and not 180°. In this specification, “equiangularly” is thus intended to mean that the angles differ by no more than 30°, preferably by no more than 10°, more preferably by no more than 5°.

Preferably, the keys are arranged in sets of two, with the two keys in a set being positioned in diametrically opposed positions in a horizontal plane about said common vertical axis of rotation.

The keys may be arranged in a circle, and may be interspaced by at least one fastener between adjacent keys, the fastener acting to inhibit or prevent vertical displacement of the upper rotatable grate component and the lower rotatable support structure. The fasteners may be in the form of nut and bolt arrangements.

Typically, the keys are unthreaded and do not act to inhibit or prevent vertical displacement of the upper rotatable grate component and the lower rotatable support structure.

Preferably, the keys are not circular cylindrical.

The keys may be tapered from a wider operatively upper end to a narrower operatively lower end or vice versa.

The keys may be elongate and may be polygonal in outline in end view, e.g. square. It is to be understood that the end view is taken in a direction along a longitudinal axis of the elongate key.

The keys may be sturdier than the fasteners so that a shank portion of a fastener has a maximum transverse dimension which is less than a minimum transverse dimension of a key.

The rotatable grate component may have an upwardly inwardly tapering outer surface. The upwardly inwardly tapering outer surface may be staggered or stepped when seen in vertical cross-section, defining vertically and radially spaced terraces. The terraces may be covered by shield plates to protect the rotatable grate component from abrasive material, e.g. ash.

According to another aspect of the invention, there is provided a gasifier for gasifying carbonaceous material, the gasifier including a grate assembly as hereinbefore described mounted within a gasification chamber defined by a gasification vessel with the lower rotatable support structure of the grate assembly being connected to drive means.

The gasifier may be a fixed bed gasifier, in particular a fixed bed dry bottom gasifier.

The gasifier may be a coal, waste or biomass gasifier, or a gasifier configured to gasify a combination of two or more of coal, waste and biomass. Typically however, the gasifier is a coal gasifier operating at an elevated pressure of between 5 bar(g) and 100 bar(g) and a temperature of between 400° C. and 1600° C.

The rotatable grate component typically has a vertical dimension and a radial direction and is rotatable about a vertical axis of an ash discharge outlet of the gasification chamber, with a lower periphery of the rotatable grate component being below an apex or upper end of the rotatable grate component.

In one embodiment of the invention, in use, the grate component is rotated about the vertical axis of said ash discharge outlet. Carbonaceous feedstock, e.g. coal, is fed batch-wise into a top of the gasifier and gasification agent is fed into a bottom of a reaction zone defined by the gasification vessel through gasification agent outlets underneath lower edges of the outer shield plates of the rotatable grate component, thereby to gasify coal located in a slow moving bed within the gasification chamber. Ash is continuously withdrawn from the bottom of the gasification chamber through an annular ash discharge passageway being provided between a wall of the gasification chamber and the rotatable grate component. Ploughs are rigidly connected to the rotatable grate component which continually rotates in operation, the ploughs scraping the ash through said ash discharge passageway, whereafter the ash flows under force of gravity into an ash lock located beneath the gasifier vessel. As the grate assembly rotates, clinker crushing is performed between the shield plates of the rotatable grate component and wear bricks located on the wall of the gasifier vessel.

The invention will now be described by way of example with reference to the accompanying diagrammatic drawings in which:

FIG. 1 shows a vertically sectioned view of a grate assembly in accordance with the invention, installed in a fixed bed dry bottom coal gasifier in accordance with the invention;

FIG. 2 shows a plan view of a lower rotatable support structure of the grate assembly of FIG. 1;

FIG. 3 shows a plan view representation of the relative rotation of the rotating grate component and the rotating support structure in an upset condition in a grate assembly of the prior art; and

FIG. 4 shows a plan view representation of the rotation of a grate assembly according to the present invention.

With reference to FIG. 1 of the drawings, reference numeral 10 generally refers to a grate assembly in accordance with the invention installed in a gasification vessel 100 of a fixed bed dry bottom coal gasifier in accordance with the invention. The gasification vessel 100 defines a gasification chamber 102 within which the grate assembly 10 is located. The gasification chamber 102 has a wall 18.

The grate assembly 10 comprises an upper rotatable grate component 11, fixedly connected to a lower rotatable support structure 12 by a plurality of connecting bolts 15 and at least two key torque transmitters 16. The lower rotatable support structure 12 is drivingly connected to a ring gear 13 which is in turn connected through a gearbox to an electric motor (not shown). In use, the electric motor and gearbox are used to rotate the rotatable components 11, 12 and 13 of the grate assembly 10.

The rotatable components 11, 12 and 13 are supported by a stationary support structure 14 above an ash discharge outlet or passageway 20 of the gasification vessel 100. One or more ploughs 17 are connected to a radially outermost periphery of the upper rotatable grate component 11. The ploughs 17 are used to scrape ash from an ash bed within the gasification chamber 102 through an ash discharge annulus 19 which is defined between the outer periphery of the upper rotatable grate component 11 and the wall 18 of the gasification chamber 102.

In use, the grate assembly 10 is rotated about an axis of rotation 21, which corresponds with the centre line of the ash discharge passageway 20 which is also the centre line of the grate assembly 10. Coal is fed batch-wise into the top of the gasifier (not shown) and gasification agent is fed into a lower portion of the gasification chamber 102 through gasification agent outlets 23 underneath the lower edges of the outer shield plates 22, thereby to gasify coal located in a slow moving bed within the gasification chamber 102. Ash is continuously withdrawn from the bottom of the gasification chamber 102 through the ash discharge annulus 19 provided between the wall 18 of the gasification chamber 102 on the one hand and the upper rotatable grate component 11 on the other hand, and through the ash discharge passageway 20. The ploughs 17 rotate with the upper rotatable grate component 11 thereby discharging ash by scraping it through the ash discharge annulus 19. As the upper rotatable grate component 11 rotates, clinker crushing is performed between the shield plates 22 of the upper rotatable grate component 11 and the wall 18 of the gasifier chamber 102.

With reference to FIG. 2 of the drawings, a plan view of the rotatable support structure 12 of the grate assembly 10 of FIG. 1 is shown. A plurality of bolts (not shown) corresponding to a plurality of bolt holes 21 are used to fixedly but removably connect the upper rotatable grate component 11 to the lower rotatable support structure 12. The bolts are designed vertically to connect the adjacent components 11 and 12 and are not designed to withstand radial shear forces. At least two diametrically opposed keyways 22 are provided in which corresponding keys (not shown) are snugly located. The keys are designed to act as torque transmitters, transferring the driving force from the electric motor and gearbox (not shown) via the ring gear 13 and the rotatable support structure 12 to the upper rotatable grate component 11.

In use, upset conditions may occur where ash agglomerates become wedged in the ash discharge annulus 19 located between the upper rotatable grate component 11 and the vessel wall 18 as shown in FIG. 1. The grate assembly 10 is designed to crush the agglomerates wedged in the ash discharge annulus 19. However, circumstances may arise where the agglomerates cannot easily be broken for removal via the ash discharge annulus 19 and ash discharge passageway 20 and the rotation of the upper rotatable grate component 11 is hindered. In such cases, the ring gear 13 continues to drivingly rotate the rotatable support structure 12 which is fixedly connected to the upper rotatable grate component 11. The rotational force is transferred to the rotatable grate component through the key torque transmitters located in the keyways 22 as described.

Investigations have revealed that, when only one key torque transmitter is used, in the event of the rotation of the upper rotational grate component 11 being restricted, a relative rotation around the key in the keyway 22 occurs, i.e. the upper rotatable grate component 11 which is restricted from rotating by the ash agglomerate tends to pivot around the key in the keyway 22. The relative movement of the two adjacent components 11, 12 around the key leads to a displacement of the connecting bolts relative to the corresponding bolt holes 21 and of the key relative to the keyway 22. This leads to a displacement of the axes of rotation, marked A and B in FIG. 3 of the drawings.

In FIG. 3, the direction of rotation is shown by arrow 30. The rotational profile of the stationary component (i.e. the upper rotatable grate component 11) is shown generally as reference numeral 110. A key (not shown) is located in the keyway 22. The rotational profile of the rotating component (i.e. rotatable support component 12) is shown generally as reference numeral 120. The axis of rotation of the profile 110 is shown as point A, whereas the axis of rotation of the profile 120 is shown as point B.

The inventors have found that, in operation, it is critical that the axes of rotation A and B of the rotatable grate components coincide, i.e. that points A and B overlap along a common axis of rotation 21 as shown in FIG. 1. In an upset condition, the axis of rotation represented by point B becomes offset from the common axis of rotation 21 and accordingly no longer overlaps with the axis of rotation represented by point A. The displacement of point B from the central axis of rotation 21 as a result of a relative rotation of the support component 12 around the key in the keyway 22, leads to a progressive increase of the shear forces acting on the connecting bolts located in the bolt holes 22. As the connecting bolts fail sequentially, the area available to transmit the driving force of the ring gear 13 diminishes and as a consequence there are fewer connecting bolts to carry the load, leading finally to the failure of all the connecting bolts. With the connecting bolts not able to prevent the relative movement of the two connected rotatable components 11 and 12, the rotatable components 11 and 12 of the grate assembly 10 separate leading to a functional failure of the grate assembly 10.

FIG. 4 shows a diagram of the rotational profiles of the rotatable grate components 11, 12 according to the present invention. The introduction of additional symmetrically spaced keys located in corresponding keyways 22 prevents any displacement of the axis of rotation, which is a common problem in the grate assembly of the prior art wherein a single key and keyway 22 is provided between the upper rotatable grate component 11 and the lower rotatable support structure 12. Thus, the inclusion of one or more sets of keys provides that the axes of rotation represented by points A and B overlap, coinciding with the axes 21, even in the upset condition described hereinbefore.

Preferably one set of keys (i.e. two keys) spaced substantially diametrically opposite one another can be used, more preferably two sets of keys (i.e. four keys) can be used. Each set of keys is spaced substantially opposite one another such that the keys are substantially orthogonal and equiangularly spaced around the circumference of the rotatable support structure 12 which attaches to the upper rotatable grate component 11. In instances where the upper rotatable grate component 11 and the lower rotatable support structure 12 do not consist of an equal number of segments (typically the components of the grate assembly 10 are constructed by joining together three or more segments to make up the part), a minimum of one key per segment is acceptable.

The additional keys will, in use, provide an additional connecting and torque transfer mechanism which will prevent the relative displacement of the axes of rotation A and B. In mitigating the displacement of the axes or rotation A and B to keep them coaxial with the axis 21, the transfer of rotational loads onto the connecting bolts is prevented and ultimately separation of the connected rotatable components 11 and 12 and functional failure of the grate assembly 10 is avoided.

The grate assembly 10 of the invention, as illustrated, thus provides for a reduction in lost production due to greater availability of equipment and a reduction in operating costs due to a reduction in maintenance required to repair functional failures of the grate assembly. Furthermore, advantageously, minor modifications only are required to retrofit the solution provided by the invention to existing equipment. In other words, retrofitting of existing grate assemblies to obtain the advantages of the invention is thus achieved by means of relatively minor modifications to the existing equipment. 

1. A grate assembly for a gasifier for gasifying carbonaceous material producing ash, the grate assembly including: an upper rotatable grate component; and a lower rotatable support structure fastened to the upper rotatable grate component by a plurality of removable fasteners acting to prevent vertical displacement of the upper rotatable grate component and the lower rotatable support structure, the lower rotatable support structure being configured to be drivingly rotated about a common vertical axis of rotation shared with the upper rotatable grate component, the assembly further including at least two key torque transmitters spaced angularly relative to said common axis of rotation and engaging the upper rotatable grate component and the lower rotatable support structure to transfer torque from the lower rotatable support structure to the upper rotatable grate component when the lower rotatable support structure is driven.
 2. The grate assembly of claim 1, wherein the key torque transmitters are equiangularly spaced in a horizontal plane about the common vertical axis of rotation.
 3. The grate assembly of claim 1 or claim 2, wherein two key torque transmitters are provided and are spaced 170-190° apart.
 4. The grate assembly of claim 1 or claim 2, wherein three key torque transmitters are provided and are spaced 110-130° apart.
 5. The grate assembly of claim 1 or claim 2, wherein the key torque transmitters are arranged in sets of two, with the two key torque transmitters in a set being positioned in diametrically opposed positions about the common vertical axis of rotation.
 6. The grate assembly of any of claims 1 to 5, wherein the key torque transmitters are arranged in a circle and are interspaced by at least one fastener between adjacent key torque transmitters, the fastener acting to inhibit or prevent vertical displacement of the upper rotatable grate component and the lower rotatable support structure.
 7. The grate assembly of claim 6, wherein the fasteners are in the form of nut and bolt arrangements.
 8. The grate assembly of any of claims 1 to 7, wherein the key torque transmitters are unthreaded and do not act to inhibit or prevent vertical displacement of the upper rotatable grate component and the lower rotatable support structure.
 9. The grate assembly of claim 8, wherein the key torque transmitters are tapered from a wider operatively upper end to a narrower operatively lower end.
 10. The grate assembly of claim 1, wherein the key torque transmitters are elongate and polygonal in outline in end view.
 11. A gasifier for gasifying carbonaceous material, the gasifier including a grate assembly as claimed in any of claims 1 to 10 mounted within a gasification chamber defined by a gasification vessel with the lower rotatable support structure of the grate assembly being connected to drive means.
 12. The gasifier of claim 11, which is a fixed bed gasifier
 13. The gasifier of claim 11 or claim 12, which is a fixed bed dry bottom gasifier.
 14. The gasifier of claim 13, which is a coal, waste or biomass gasifier, or a gasifier configured to gasify a combination of two or more of coal, waste and biomass.
 15. The gasifier of claim 14, which is a coal gasifier operating at an elevated pressure of between 5 bar(g) and 100 bar(g) and a temperature of between 400° C. and 1600° C. 